How do I eliminate a split screen?

When I open my Firefox browser, two websites open. One on the left side of the screen, which can be expanded to about 1/4 of the entire screen and the other site opens on the remaining portion of the screen. I can close the left site with by clicking on the x, but the next time I open the browser both appear again.

How can I change this so that only one site opens at a time?

When I open my Firefox browser, two websites open. One on the left side of the screen, which can be expanded to about 1/4 of the entire screen and the other site opens on the remaining portion of the screen. I can close the left site with by clicking on the x, but the next time I open the browser both appear again.
How can I change this so that only one site opens at a time?

The site opening on the left side of the screen is opening in the sidebar. Locate that bookmark, right-click on it, choose Properties, un-check "Load this bookmark in the sidebar", click Save. Restart Firefox and see if the problem is solved.

The site opening on the left side of the screen is opening in the sidebar. Locate that bookmark, right-click on it, choose Properties, un-check "Load this bookmark in the sidebar", click Save. Restart Firefox and see if the problem is solved.
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Check your plugins here: http://www.mozilla.com/en-US/plugincheck/
It appears you have 2 plugins with security issues and they need to be updated:
*''Update Java''': http://support.mozilla.com/en-US/kb/Using+the+Java+plugin+with+Firefox
*'''Update Adobe Reader (Adobe PDF plugin)''': http://support.mozilla.com/en-US/kb/Using+the+Adobe+Reader+plugin+with+Firefox

I tried again and this time I got the Properties dialogue box, and unchecked "show in the side bar". Then I hit save, closed Firefox, reopened and nothing had changed. I did it again and then rebooted, still the same problem. Got any other ideas?

I tried again and this time I got the Properties dialogue box, and unchecked "show in the side bar". Then I hit save, closed Firefox, reopened and nothing had changed. I did it again and then rebooted, still the same problem. Got any other ideas?

You can check that by opening the bookmark in the main window and click the star on the location bar.

Opening in the side bar is the default for bookmarks that are created via a link or button on a website.
You can check the Properties of a bookmark via the right-click context menu in the side bar (Ctrl+B; Cmd+B on Mac).
In the Bookmarks Manager (Bookmarks > Show All Bookmarks) you can click the More button in the Details pane at the bottom right.
Make sure that "Load this bookmark in the side bar" is not selected.

You may have more copies of that bookmark.
You can check that by opening the bookmark in the main window and click the star on the location bar.
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Opening in the side bar is the default for bookmarks that are created via a link or button on a website.<br />
You can check the Properties of a bookmark via the right-click context menu in the side bar (Ctrl+B; Cmd+B on Mac).<br />
In the Bookmarks Manager (Bookmarks > Show All Bookmarks) you can click the More button in the Details pane at the bottom right.<br />
Make sure that "Load this bookmark in the side bar" is not selected.<br />